<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="color" name="" id="theme">
    <button>设置主题</button>

    <!-- 更改主题颜色 -->
    <script>
        // 获取元素
        const btn = document.querySelector('button');
        const theme = document.querySelector('#theme');

        // 改变主题背景
        btn.onclick = () => theme.value && (document.body.style.backgroundColor = theme.value);
    </script>
</body>

</html>